Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and

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Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and Dinuclear Hydroxide Complexes and Ligand Rollover C−H Activation Wilson D. Bailey,† Lapo Luconi,‡ Andrea Rossin,‡ Dmitry Yakhvarov,§ Sarah E. Flowers,† Werner Kaminsky,† Richard A. Kemp,*,∥,⊥ Giuliano Giambastiani,*,‡,§ and Karen I. Goldberg*,† †

Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States Institute of Chemistry of Organometallic Compounds ICCOM-CNR and Consorzio INSTM, Via Madonna del Piano 10, 50019 Sesto F.no Florence, Italy § Kazan Federal University, 420008 Kazan, Russian Federation ∥ Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States ⊥ Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, United States ‡

S Supporting Information *

ABSTRACT: Palladium complexes of the novel unsymmetrical phosphine pyrazole-containing pincer ligands PCNH (PCNH = 1-[3[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCNMe (PCNMe = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1Hpyrazole) have been prepared and characterized through single-crystal X-ray diffraction and multinuclear 1H, 13C{1H}, and 31P{1H} NMR spectroscopy. In preparations of the monomeric hydroxide species (PCNH)Pd(OH), an unexpected N detachment followed by C−H activation on the heterocycle 5-position took place resulting in conversion of the monoanionic {P,C−,N} framework into a dianionic {P,C−,C−} ligand set. The dinuclear hydroxide-bridged species (PCNH)Pd(μ-OH)Pd(PCC) was the final product obtained under ambient conditions. The “rollover” activation was followed via 31P{1H} NMR spectroscopy, and dinuclear cationic μ-OH and monomeric PdII hydroxide intermediates were identified. DFT computational analysis of the process (M06//6-31G*, THF) showed that the energy barriers for the pyrazolyl rollover and for C−H activation through a σ-bond metathesis reaction are low enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCNH system, no “rollover” reactivity was observed in the PCNMe system, and the terminal hydroxide complex (PCNMe)Pd(OH) could be readily isolated and fully characterized.

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greater numbers (NCC, PNN, PCO, PCS, etc.).6 PCN-type systems in particular are intriguing, because in (PCN)M(L)n complexes (M = transition metal; L = ancillary ligand) the tridentate hybrid ligand contains both hard (N) and soft (P) donor functions, thus leading to novel and unprecedented chemical properties.7 In such species, there is a marked difference in the trans effect between the two different donor arms. This difference results in the group with the weaker trans effect (N) being more likely to dissociate from the metal center due to its position trans to the group with a stronger trans effect (P). The stronger M−P bond remains intact. From a homogeneous catalysis perspective, the hemilability of the ligand provides access to a vacant coordination site at the metal center and so can allow for effective coordination, activation, and transformation of substrate molecules. Such ligand hemilability has often been invoked as the critical factor when comparing the catalytic activity of (PCP)M(L)n/

INTRODUCTION The use of pincer-ligated transition-metal complexes as catalysts for organic transformations has grown dramatically in recent years.1 The robust tridentate binding motif coupled with the tunability of the steric and electronic parameters of pincer ligands has proven highly effective in stabilizing and allowing isolation of a variety of uncommon types of metal complexes. For example, pincer ligands have been very useful in the preparation of mononuclear late-metal complexes bearing M−OR and M−NR2 bonds. Notably, there are significantly fewer mononuclear late-transition-metal hydroxide, alkoxide, and amide complexes relative to their metal alkyl (M−C) analogues.2 However, such M−OR and M−NR2 linkages are pertinent to catalysis,3,4 and thus isolation and study of model metal hydroxide, alkoxide and amide complexes are of great value. The variety of available pincer ligands has increased in recent years. While early pincer ligands were symmetric with respect to ligand “arms” (e.g., PCP, PNP, POCOP, etc.),5 pincer-type complexes bearing unsymmetrical arms have begun to appear in © XXXX American Chemical Society

Received: April 30, 2015

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DOI: 10.1021/acs.organomet.5b00355 Organometallics XXXX, XXX, XXX−XXX

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available (3-bromophenyl)methanol. The phosphine group was introduced by transformation of the benzyl alcohol moiety into a better leaving group (reaction ii, bromination) followed by the phosphine nucleophilic substitution (reaction iii) as the final synthetic step. The PCNH pincer ligand 3a was isolated as an analytically pure and air-sensitive white solid in 48% yield from 3-bromobenzyl alcohol (see the Experimental Section). Synthesis of Chloro, Nitrile, and Triflate Complexes Containing the [(PCNH)Pd]+ Fragment. The reaction of PCNH ligand 3a with Pd(COD)Cl2 in toluene (110 °C) was monitored through successive solution samplings and 31P{1H} NMR spectroscopy at room temperature. Full conversion to the PdII complex (PCNH)PdCl (4a) was observed after 4 h with a new downfield 31P{1H} NMR signal at 95.1 ppm (Scheme 2). The (PCNH)PdCl complex 4a was isolated as a moderately air sensitive white microcrystalline solid and was characterized by multinuclear 31P{1H}, 1H, and 13C{1H} NMR spectroscopy combined with single-crystal X-ray diffraction studies. The 1H and 13C{1H} NMR patterns indicate that 4a possesses Cs symmetry in solution. The most representative 1H and 13 C{1H} NMR spectroscopic resonances related to the κ3coordinated ligand fall at lower fields in comparison with those observed for the free ligand; the aryl carbon atom directly bound to the PdII center shows the largest resonance shift (from 120.1 (3a) to 150.3 (4a) ppm for PCNH). Microcrystals suitable for X-ray analysis were grown from a concentrated acetone solution at −30 °C. An ORTEP representation of the crystal structure of 4a is given in Figure 1. Table S1 (Supporting Information) gives all the main crystal and structural refinement data, and selected bond lengths and angles are summarized in Table S2 (Supporting Information). Complex 4a crystallizes in the orthorhombic P212121 space group with four molecules per unit cell. The PdII center adopts a distorted-square-planar coordination geometry (τ4 = 0.16).13 Bond lengths and angles measured within the [(PCNH)Pd]+ fragment of 4a fall in the typical range observed for related [(PCNH)Pd]+ fragments in square-planar environments.14,9b It is notable that the Pd−P bond in 4a (2.227(4) Å) is shorter than those observed in symmetric (PCP)Pd(L) analogues bearing the −PtBu2 group (mean Cambridge Structural Database (CSD) value 2.31 Å)10,15 and that the Pd−N bond length in 4a (2.094(10) Å) is longer than that found in symmetric pyrazole-based (NCN)Pd(L) species (mean CSD value 2.03 Å).16 This crystallographic evidence suggests that the pyrazolyl group is more likely to possess hemilabile properties in the unsymmetrical PCNH environment. It was found that the chloride ligand trans to the strongly donating aryl backbone in 4a could be abstracted by treatment of 4a with silver reagents (Scheme 2). The nitrile complex [(PCNH)Pd(MeCN)](BF4) (5a) was obtained as a finely divided white powder from the reaction of 4a with 1 equiv of AgBF4 in CH2Cl2 in the presence of a 5-fold excess (compared to Pd) of acetonitrile. Complex 5a was characterized by multinuclear NMR spectroscopy and elemental analysis. Unfortunately, attempts to obtain crystals of 5a suitable for X-ray analysis were unsuccessful. The 1H and 13C{1H} NMR resonances of 5a are very similar to those observed for the parent chloride precursor 4a, except for the appearance of a sharp singlet in the 1H NMR spectrum at 2.51 ppm (ascribed to the methyl group of the coordinated CH3CN molecule). The 31P{1H} NMR signal moves from 95.1 ppm for 4a to 98.2 ppm for 5a.

(NCN)M(L)n and (PCN)M(L)n analogues; the unsymmetrical complex has consistently been shown to be more active than its symmetric counterparts.6i,j,8,9 In our laboratories, we have explored a variety of both symmetric and unsymmetrical pincer systems (PCP,10 PNP,11 PCO,6h NNC12) to stabilize and study model complexes. While the symmetric ligand systems result in more stable complexes, the unsymmetrical analogues have yielded higher reactivity primarily through arm lability. In a study on late-transitionmetal PCO pincer systems, the ether O arm was found to be labile and allowed for reductive elimination of the pincer framework under reducing conditions.6h Ligand arm aromatic/ heteroaromatic C−H activation through σ-bond metathesis was observed in early-transition-metal and rare-earth complexes to give unsymmetrical NNC pincer complexes.12 The following is an account of a novel unsymmetrical PCN pincer ligand system designed to relieve steric bulk around the metal center relative to its PCP counterpart. The nitrogen donor (pyrazolyl group) was expected to be a more strongly binding ligand than an ether linkage (as in a related PCO system) yet would potentially provide for hemilability at elevated temperatures. Notably, this is also a cautionary tale about unsymmetrical pincer ligands. Pincer ligands are often touted for their robustness, but here we find that the unsymmetrical members of the family can display unwanted reactivity of their own. The metalation of two new pyrazolecontaining PCN ligand frameworks on PdII and the exploration of the reactivity of the related chloride and hydroxide complexes are discussed. In a hydroxide derivative, an unexpected pyrazolyl side arm C−H bond activation was observed. C−H addition across the Pd−OH bond on the 5position of the heterocycle occurred to transform the monoanionic {P,C−,N} ligand into a dianionic {P,C−,C−} donor. The thermodynamics are favorable, with the driving force for the reaction to occur provided by the simultaneous formation of a stable neutral aquo ligand that exits the metal coordination sphere. Ligand loss generates a positive entropy change and an overall (more) negative Gibbs energy variation for the side arm C−H activation reaction. The process was followed experimentally (multinuclear 31P{1H} and 1H NMR spectroscopy) and then computationally modeled (DFT at the M06//6-31G* level of theory). By protection of the ligand in the 5-position through methylation (PCNMe), this “rollover” reactivity was shut down and a stable terminal hydroxide complex was isolated.

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RESULTS AND DISCUSSION Synthesis of the PCN-Pincer Ligand PCNH (3a). Scheme 1 summarizes the synthetic path used to obtain the PCNH pincer ligand 3a in fairly good yields starting from commercially Scheme 1. Synthesis of the Pincer PCNH Ligand 3aa

a

Reagents and conditions: (i) pyrazole, CuI, K2CO3, NMP, microwave irradiation, 210 °C, 5 h, 250 W; (ii) Br3CCO2Et, PPh3, CH2Cl2, room temperature, 0.5 h; (iii) tBu2PH, acetone, reflux, 12 h. B


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Organometallics Scheme 2. Synthesis of the Chloride (4a, 6a), Nitrile (5a), and Triflate (7a) Pincer Complexes of PCNH

Figure 2. ORTEP representation of the cationic part of the salt {[(PCNH)Pd]2(μ-Cl)}BF4 (6a), with ellipsoids shown at 50% probability. Hydrogen atoms, tBu groups on P, the BF4− counterion, and crystallization solvent molecules are omitted for clarity.

Figure 1. ORTEP representation of (PCNH)PdCl (4a), with ellipsoids shown at 50% probability. Hydrogen atoms are omitted for clarity.

Treatment of a CH2Cl2 solution of 4a with only 0.5 equiv of AgBF4 (Scheme 2) afforded the dinuclear PdII complex {[(PCNH)Pd]2(μ-Cl)}BF4 (6a), in the form of pale yellow microcrystals. The NMR spectra of the chloride-bridged dimer show the classical patterns of C2-symmetric species in solution, whose most representative features are given by two sharp doublets in the 13C{1H} NMR spectra: one coming from the 12 methyl groups (28.8 ppm, JPC = 3.5 Hz) of the tert-butyl moieties and the other belonging to the ipso carbon atom σbound to the palladium center (149.8 ppm, JPC = 13.3 Hz). As expected, a downfield sharp singlet is also observed in the 31 1 P{ H} NMR spectrum (95.1 ppm). Microcrystals suitable for X-ray diffraction were grown by layering a concentrated CH2Cl2 solution of the salt with cold pentane. The solid-state structure confirmed the identity of 6a (an ORTEP representation is given in Figure 2, and relevant bond angles and distances are given in Table S2 (Supporting Information)). The dimer crystallizes in the orthorhombic Pna21 space group, with four molecules per unit cell. In the chloride-bridged dinuclear cation, both palladium centers maintain a slightly distorted square planar geometry. The main bond lengths and angles in 6a are very similar to those found in the monomeric complex 4a. Similarly to related μ-Cl monobridged cationic

dimers,17 only slightly longer Pd−Cl distances are found in 6a (2.427(4) Å for Pd(1)−Cl(1) and 2.425(4) Å for Pd(2)− Cl(1)) in comparison to the bond length determined in 4a (2.388(4) Å). The reaction of 4a with 1 equiv. of AgOTf in CH2Cl2 in the absence of acetonitrile yields the triflate species (PCNH)Pd(OTf) (7a) (Scheme 2). Complex 7a was fully characterized by multinuclear NMR spectroscopy, X-ray crystallography, and combustion analysis. The 1H and 13C{1H} NMR spectra display patterns very similar (albeit slightly shifted) to those observed for the chloride analogue 4a. The 31P{1H} NMR spectrum displayed a singlet at 94.1 ppm. Complex 7a exhibits a typical square-planar ligand arrangement about the Pd center with κ3-PCNH and triflate ligands coordinated. An ORTEP representation of 7a is shown in Figure S2 (Supporting Information). Relevant bond lengths and angles appear in Table S2 (Supporting Information). Formation of Hydroxide Species Containing the [(PCNH)Pd]+ and [(PCC)Pd] Fragments: “Rollover” C−H Bond Activation on the Pyrazolyl Side Arm. As previous transition-metal hydroxide complexes have been synthesized primarily through metathesis reactions with alkali-metal C


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Organometallics hydroxides,18 preparation of the desired monomeric hydroxide complex was first attempted by addition of potassium hydroxide (KOH) to the chloride complex 4a in tetrahydrofuran (THF). No reaction was observed at room temperature, and only a slow reaction occurred to produce an array of intractable products, including palladium black, over very long reaction times (4 days) at elevated temperatures (60 °C). As group 10 terminal hydroxides have also been made via metathesis of weakly bound anions rather than from the chloride precursor,19 the reactivity of the triflate complex 7a with hydroxide salts was investigated. Addition of an excess of KOH to a THF-d8 solution of the triflate complex 7a yielded the new product 8a, as observed by the disappearance of the signal at 94.1 ppm and the appearance of a new singlet at 93.3 ppm in the 31P{1H} NMR spectra (Figure 3). However, further monitoring of this reaction

temperature. In this case, reversible cleavage of the dinuclear complex 6a may be occurring on the NMR time scale, resulting in the tBu and methylene signals appearing as respectively averaged signals. Another possibility is that free rotation about the Pd−Cl bond of 6a occurs at room temperature, although this option may be considered less likely given the similar steric profiles of 6a and 8a. X-ray-quality crystals of 8a were grown from a concentrated THF solution layered with pentane. The solid-state structure confirmed the spectroscopic assignment of 8a as the dinuclear species {[(PCNH)Pd]2(μ-OH)}(OTf) (Figure 4).

Figure 4. ORTEP representation of the cationic part of the salt {[(PCNH)Pd]2(μ-OH)}(OTf) (8a), with ellipsoids shown at 50% probability. Hydrogen atoms, tBu groups on P, and triflate counterion are omitted for clarity. Figure 3. 31P{1H} NMR stack of the formation of species 8a−10a over days.

Dinuclear 8a is C2 symmetric with the two distorted-squareplanar (PCNH)Pd units bridged by a single μ-OH (τ4[Pd(1)] = 0.14; τ4[Pd(2)] = 0.16).13 Each palladium is formally PdII; one noncoordinating triflate counterion is found in the asymmetric unit to balance the overall positive charge of the dinuclear complex. The structure of 8a is analogous to that of the chloride-bridged dinuclear species 6a, with a bridging hydroxide instead of a bridging chloride. The hydroxyl hydrogen was refined by geometry optimization of the hydroxide ligand with respect to the Pd(1)−O(1) bond, while leaving the Pd(2)− O(1) bond free. The hydroxide is shielded by the bulky tBu groups, and no hydrogen-bonding interactions were observed. The bond lengths within the (PCNH)Pd units of 8a are similar to those observed in the mononuclear triflate complex 7a. While bis-μ-OH dimers are well-known for Pd, this is a rare example of two palladium centers bridged only by a single μOH ligand.20 Table S2 in the Supporting Information contains selected bond lengths and angles for 8a. When a THF solution of the triflate 7a was treated with a full 1 equiv of KOH, 8a was observed as a transient intermediate and decayed into complex 9a as the major product. Complex 9a is characterized by a singlet at 91.8 ppm in the 31P{1H} NMR spectrum. Conversion to complex 9a was not clean, and peaks at 91.1 and 71.0 ppm (complex 10a) in the 31P{1H} NMR

showed conversion of 8a to the second species 9a with a singlet at 91.8 ppm in the 31P{1H} NMR spectrum. A third product 10a, with two new signals at 91.1 and 71.0 ppm, then grows in concurrently with a relative decrease of the 91.8 ppm signal due to 9a. Some decomposition of the reaction mixture was also observed in the reaction vessel as palladium black became visible. When the reaction conditions were changed as described below, characterization of the various palladium species formed in this reaction (8a−10a) was possible. When only 0.5 equiv of KOH was added to the triflate 7a, the new species 8a was formed as the sole product (Scheme 3). The 1H NMR spectrum of 8a displays a peak at −1.48 ppm, indicative of a Pd hydroxide species. The tBu protons appear as two doublets at 1.44 and 1.51 ppm (3JPH = 14.1 and 14.3 Hz, respectively). Integration of the doublets in comparison to the hydroxide signal yields a 18:18:1 ratio, suggesting a dinuclear species. The appearance of two signals for the tBu protons demonstrates that these positions are diastereotopic, as would be expected in a rigid nonplanar dinuclear complex. This observation is in contrast with what is found for 6a, where only one resonance is observed for the tBu groups at room

Scheme 3. Synthesis of the Hydroxide Pincer Complexes 8a−10a of the Ligand PCNH

D


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Organometallics

lengths within 10a are compared, a clear difference is observed. In 10a, the hydroxide ligand is associated more with the cationic [(PCN)Pd]+ fragment (2.0831(11) Å) than with the neutral [(PCC)Pd] fragment (2.1077(11) Å), further supporting a mixed-ligand dinuclear species. It should be noted that, for complex 8a, the hydroxide ligand is shared equally between palladium centers within error. The activation of a pyrazolyl C−H bond at the 5-position has previously been observed in (poly)pyrazolyl-containing ligands in the presence of late transition metals (e.g., Ru,21 Pd,22 Pt,23 and Ir24), and the activation of C−H bonds by palladium fragments in several N-heterocycles has been studied computationally.25 Notably, however, the “rollover” activation to form 10a takes place under very mild conditions, in contrast to many previously reported literature examples that require elevated temperatures or the employment of photoirradiation. We propose that the room-temperature “rollover” C−H activation occurs at the monomeric hydroxide 9a, forming a neutral and coordinatively unsaturated [(PCC)Pd] unit after water release (i.e., loss of an aquo ligand from the palladium coordination sphere). This is supported by the increased stability of 9a in the presence of water (vide supra). Subsequent combination of the [(PCC)Pd] fragment with another molecule of (yet unreacted) 9a yields the bridged species 10a (Scheme 3). When a THF solution of a mix of 9a as the major product and 10a as a minor impurity was left at room temperature for 6 days, decomposition of 9a occurred, yielding 10a as the major product, along with unidentified side products as observed by multinuclear NMR spectroscopy. Attempts to obtain a pure sample of 10a in solution were unsuccessful. However, by analyzing a mixture of 9a converting to 10a before further decomposition, signals attributed to the mixed ligand complex 10a could be, in part, made by NMR spectroscopy. Notably, the 1H NMR spectrum of the mixture shows four new tBu signals (1.36, 1.39, 1.44, and 1.51 ppm), each as doublets, indicating that the substituents are not only diastereotopic as in complex 8a but are bound to two chemically inequivalent phosphorus atoms. This is expected, as one phosphine arm is trans to N and the other trans to C. This mixed-ligand framework is also observed by four sets of doublets of doublets for the diastereotopic methylene protons (3.08, 3.12, 3.42, and 3.49 ppm), as well as four tBu primary carbon signals in the 13 C{1H} NMR spectrum (29.46, 29.51, 29.91, and 30.10 ppm; 2 JPC = 3.9, 3.5, 6.0, and 6.0 Hz, respectively). Unfortunately, due to the complexity of the mixture and the inability to isolate 10a, the other resonances in the 13C{1H} NMR spectrum could not be assigned confidently. To explore this “rollover” activation in more detail, DFT simulations of the reaction were undertaken. DFT Simulation of the “Rollover” Cyclometalation in the Conversion of the Mononuclear Hydroxide 9a to the Dinuclear μ-OH Complex 10a. The pyrazolyl “rollover” and related activation of the C(5)−H pyrazolyl bond to form H2O was examined computationally on the real system at the M06// 6-31G* level of theory. THF solvent was included in the calculation through a continuum model (SMD), and all the figures reported here should be considered as ΔG or ΔG⧧ values evaluated in THF (see Computational Details). A water molecule generated by an intermolecular rearrangement of the hydroxide complex 9a implies a (PCNH)− ↔ (PCC)2− conversion, which is proposed on the basis of the experimental results wherein the dinuclear complex (PCNH)Pd(μ-OH)Pd(PCC) (10a) was isolated. The rotation of the pyrazolyl ring

spectrum consistently appeared as a minor product, such that it was not possible to isolate pure 9a. However, it was found that the addition of water to 9a inhibited decomposition to 10a. Thus, addition of D2O to a THF-d8 (1/4 v/v) solution of 8a containing suspended KOH resulted in the formation of pure 9a, which could be fully characterized in solution by NMR spectroscopy. On the basis of the intermediacy of 8a in the formation of 9a and the 1H and 31P{1H} NMR spectral signals assignable to 9a, complex 9a is proposed to be the mononuclear terminal hydroxide (PCNH)Pd(OH) (Scheme 3). The 1H NMR spectrum of 9a displays a signal at −1.71 ppm (observed only in the absence of excess water), attributable to the Pd−OH group, and the tBu protons appear as a doublet at 1.43 ppm, integrating in the ratio 18H:1H in comparison to the hydroxide signal. Efforts to recrystallize 9a from the reaction mixture in THF at low temperatures did yield crystals suitable for an X-ray diffraction study, as well as amorphous material. However, as shown in Figure 5, the crystallized product was not 9a but

Figure 5. ORTEP representation of the [(PCNH)Pd](μ-OH)[Pd(PCC)] dimer (10a), with ellipsoids shown at 50% probability. Hydrogen atoms on the pincer ligands and tBu groups on P are omitted for clarity.

rather complex 10a, the mixed-ligand hydroxide-bridged dinuclear complex [(PCNH)Pd](μ-OH)[Pd(PCC)]. This was confirmed, as dissolution of the isolated crystalline sample in THF yielded spectral features that matched those displayed by 10a in the 31P{1H} NMR spectrum along with minor impurities attributed to the amorphous solid. The structure of 10a consists of two palladium centers bridged by a single μ-OH ligand, similar to the case for 8a. One of the PdII centers is bound by the {P,C−,N} ligand as observed in 8a. However, unlike 8a, the other PdII center is bound to a dianionic tridentate {P,C−,C−} ligand; the pyrazolyl arm has “rolled over” with C−H activation having occurred at the 5-position. Details on how the pyrazolyl ring and the “rollover” ring were distinguished in the solid-state structure are presented in the Supporting Information. The [(PCNH)Pd]+ fragment displays bond angles and distances nearly identical with those found in the hydroxide complex 8a. In contrast, the Pd(1)−P(1) bond in the [(PCC)Pd] fragment is somewhat longer than the corresponding bond in the (PCNH)Pd fragment (2.3084(4) Å vs 2.2260(4) Å). This change is expected, as the phosphorus donor is trans to a stronger donor when moving from neutral N to anionic C−. Notably, no counterion was observed, which is consistent with the dianionic PCC character of the second pincer fragment. The Pd−O bond lengths in the cationic and neutral μhydroxide dimers (8a and 10a, respectively) are very similar. However, when the Pd(1)−O(1) and the Pd(2)−O(1) bond E


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Organometallics around the C(2)−N(2) bond (Figure 1 for atom numbering) within the pincer can lead to an activation of the C−H bond in the 5-position by the palladium center, with concomitant formation of the aquo species (PCC)Pd(H2O) (11a) as the result of a proton transfer to the −OH group on palladium. Beginning with the mononuclear hydroxide complex 9a, rotation of the pyrazolyl substituent by 180° with respect to the starting geometry yields an isomeric form of the hydroxide (PCCH)Pd(OH) (9a′), wherein the C−H bond in the 5position interacts with the metal center through an agostic interaction (optimized d[C−Pd] = 2.58 Å; d[H−Pd] = 2.29 Å; d[C−H] = 1.09 Å). This optimized structure is drawn in Figure 6. Unsurprisingly, N-coordination is more stabilizing than a C− H agostic interaction and the Gibbs energy of 9a′ is higher than that of 9a by 17.0 kcal mol−1.

Figure 7. Optimized structure of TS1. Selected bond lengths and distances (Å) are reported. H atoms on the pincer are omitted for clarity. For the atom color code, see the caption to Figure 6.

Figure 8. Optimized structure of (PCC)Pd(H2O) (11a). Selected bond lengths and distances (Å) are reported. H atoms on the pincer are omitted for clarity. For the atom color code, see the caption to Figure 6. Figure 6. Optimized structure of (PCCH)Pd(OH) (9a′). Selected bond lengths and distances (Å) reported. H atoms on the pincer omitted for clarity. Atom color code: gray, C; white, H; red, O; blue, N; purple, P; orange, Pd.

H2O] = 1.83 Å) and it is responsible for an extra stabilization of the dinuclear product. The optimized structure of 10a···H2O is reported in Figure 9. Evolution to the dimeric product provides

An estimation of the rotation barrier of the pyrazolyl ring was made through a scan of the θ[C(1)−C(2)−N(2)−N(1)] dihedral angle reaction coordinate (Scheme 4 and Figure 1 for Scheme 4. Pincer Rearrangement around the Pd Center through 180° Rotation of the Pyrazolyl Ring

Figure 9. Optimized structure of [(PCNH)Pd(μ-OH)Pd(PCC)]··· H2O (10a···H2O). Selected bond lengths and distances (Å) are reported. H atoms on the pincer and tert-butyl groups on phosphorus are omitted for clarity. For the atom color code, see the caption to Figure 6.

atom numbering). The maximum of the energy profile along this coordinate is located at 24.5 kcal mol−1 from the starting geometry (at θ(C−C−N−N) ≈ 75°), and it can be reasonably considered the “transition state” for this ligand rearrangement (Figure S54, Supporting Information). From 9a′, a direct proton transfer through a four-centered transition state (TS1, Figure 7;ΔG⧧ = 20.8 kcal mol−1) leads to the aquo complex 11a (Figure 8; ΔG(9a′/11a) = −11.4 kcal mol−1). The dimer formation reaction 9a + 11a → 10a···H2O is thermodynamically favored with ΔG = −12.0 kcal mol−1. In the final structure, the water molecule engages in a hydrogen bond with the bridging hydroxide group (optimized d[μ-HO···

the strong driving force for the reaction to take place. Interestingly, the pyrazolyl rotation and the C−H activation barriers are almost identical at the computational level used; thus, the heterocycle rotation (albeit slow at ambient temperature, owing to the relatively high ΔG⧧ found) is accompanied by a simultaneous C−H activation on the 5position and dimer formation, as found experimentally. Notably, this analysis is consistent with experimental results, as the aquo species 11a was never detected in the course of the F


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Organometallics transformation of 9a to 10a by 31P{1H} NMR spectroscopy. The overall Gibbs energy vs reaction coordinate profile for this transformation is reported in Scheme 5.

Scheme 7. Synthesis of the Chloride (4b) and Triflate (7b) Pincer Complexes of PCNMe

Scheme 5. Gibbs Energy (THF) vs Reaction Coordinate Profile for the Formation of the Dinuclear Species 10a···H2O Starting from the Hydroxide Complex 9a

NMR spectroscopy as well as by single-crystal X-ray diffraction. Similar to its PCNH analogue 4a, complex 4b displays Cs symmetry in solution. The spectroscopic signals for 4b are nearly identical with those of 4a, with the exception of the appearance of a methyl signal in the 1H and 13C{1H} NMR spectra at 2.71 and 13.9 ppm, respectively, and a corresponding loss of the pyz-H signal at the 5-position. A singlet at 93.3 ppm is observed in the 31P{1H} NMR spectrum. An ORTEP representation of the solid-state structure of 4b is presented in Figure S1 in the Supporting Information. The bond distances and angles of 4b are very similar to those of 4a. When 4b was treated with 1 equiv of silver triflate in methylene chloride at room temperature, AgCl precipitation occurred over 5 h and the PdII triflate complex (PCNMe)Pd(OTf) (7b, Scheme 7) was formed. Complex 7b was characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. The 1H and 13C{1H} NMR spectra of 7b are very similar to those of the nonmethylated analogue 7a, differing in the appearance of a methyl resonance at 2.68 and 14.56 ppm, respectively. The 31P{1H} NMR spectrum of 7b displays a singlet at 92.1 ppm. Similar to the case for 7a, the solid-state structure of 7b shows a square-planar complex with the triflate ligand directly bound to the Pd center (Figure S3 in the Supporting Information). The physical metrics of 7b are closely related to those of the PCNH analogue 7a, diverging with respect to the Pd−N interaction; a shorter Pd−N distance of 2.0779(16) Å is found in 7b in comparison to 2.112(2) Å in 7a. Table S2 in the Supporting Information gives a selection of bond lengths and angles in the X-ray structure of 7b. When 0.5 equiv of KOH was added to a solution of 7b in THF (Scheme 8), formation of the new species 8b was

Synthesis of the Protected PCNMe Ligand and the Corresponding Species of the [(PCNMe)Pd]+ Fragment. Conversion of the mononuclear hydroxide complex 9a to the rollover cyclometalated dinuclear 10a should be inhibited if the 5-position of the pyrazolyl ring is blocked. With this in mind, the ligand PCNMe (3b), which bears a methyl group at the 5position of the pyrazolyl, was prepared (Scheme 6). The Scheme 6. Synthesis of the Pincer PCNMe Ligand 3ba

Scheme 8. Synthesis of the Hydroxide (8b, 9b) Pincer Complexes of PCNMea

a

Reagents and conditions: (i) NaBH4, EtOH, room temperature, 1 h; (ii) Br3CCO2Et, PPh3, CH2Cl2, room temperature, 0.5 h; (iii) tBu2PH, acetone, reflux, 12 h. a

PCNMe ligand 3b was synthesized following a similar preparation from the PCNH ligand 3a, but starting from the previously reported 3-(5-methyl-1H-pyrazol-1-yl)benzaldehyde.6b The phosphine arm was installed from the benzyl bromide derivative, which after workup gave the analytically pure off-white viscous and air-sensitive oil 3b in 52% isolated yields from the aldehyde starting complex, as shown in Scheme 6 (see the Experimental Section). Metalation of 3b onto PdII to form the chloride complex (PCNMe)PdCl (4b) was carried out in a manner analogous to the metalation of 3a (vide supra and Scheme 7). Complex 4b was characterized by multinuclear 31P{1H}, 1H, and 13C{1H}

Reagents and conditions: (i) 0.5 KOH, THF, room temperature.

observed by 31 P{1 H} NMR spectroscopy through the appearance of a new singlet at 91.6 ppm. This chemical shift variation (from the triflate species 7b at 92.1 ppm) was very similar to the shift observed on going from 7a to 8a in the PCNH system, suggesting that a similar μ-OH species had formed. Indeed, 8a was identified as the symmetric dinuclear species {[(PCNMe)Pd]2(μ-OH)}(OTf) by NMR spectroscopy and single-crystal X-ray diffraction. The 1H NMR spectrum shows a diagnostic Pd−OH signal at −1.46 ppm, and integration of the 1H NMR signals was consistent with the G


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Organometallics presence of 2 equiv of PCNMe ligand per hydroxide group. Similar to 8a, the tBu protons in 8b are diastereotopic and appear as two doublets at 1.43 and 1.49 ppm (3JPH = 14.0 and 14.2 Hz, respectively). The solid-state structure (Figure S4 in the Supporting Information) displays two nearly identical [(PCNMe)Pd]+ fragments equally sharing the bridging hydroxide ligand. An outer-sphere triflate counterion is associated with the dinuclear species. While the Pd−N distance of 8b is comparable to that of 8a, the Pd−P bond of 8b (2.2381(11) Å) is lengthened in comparison to 8a (2.218(2) Å). Otherwise, the physical parameters in the solid state (bond angles and distances) between 8b and 8a are nearly indistinguishable. Addition of a full 1 equiv of KOH to the bridging dinuclear hydroxide complex 8b, or addition of an excess of KOH to the mononuclear triflate complex 7b, cleanly yielded the new product 9b, as observed by NMR spectroscopy (Scheme 8). A singlet at 90.2 ppm appeared with no additional signals in the 31 1 P{ H} NMR spectrum. The similarity of the reaction and the chemical shift change to that found for the PCNH analogue suggests that 9b is a monomeric PdII hydroxide complex. Indeed, a new signal was observed at −1.97 ppm in the 1H NMR spectrum, corresponding to a palladium hydroxide group. The upfield signal integrated to 1 equiv relative to the PCNMe signals. The 13C{1H} NMR signals for this complex were also consistent with the assignment of 9b as (PCNMe)Pd(OH). While the reaction of the palladium triflate complex 7b or the dinuclear bridging hydroxide complex 8b with KOH produced 9b, it was also determined that the same complex could be prepared directly from the chloride 4b. Thus, a metathesis reaction between 4b and an excess of KOH in THF, performed with sonication, yielded 9b directly with no observation of a bridging species such as 8b. As hypothesized, the mononuclear hydroxide complex 9b was not found to undergo further reaction (i.e. “rollover” cyclometalation) as was observed for the PCNH analogue 9a. Thus, it was possible to purify complex 9b through crystallization, and an X-ray diffraction study was used to confirm the solid-state structure (Figure 10). Complex 9b crystallizes in the orthorhombic P21212 space group, with two monomers, two water molecules, and three disordered THF solvent molecules per asymmetric unit. The square-planar PdII center is bound by the tridentate PCNMe ligand with a hydroxide ligand occupying the fourth

coordination site. Each complex is hydrogen-bound to a symmetry-related copy of itself via two water molecules and the hydroxyl ligand. This water-bridged mode has been observed previously in terminal hydroxides.19,26 The [(PCNMe)Pd]+ fragment has metrics (bond angles and distances) similar to those of the other monomeric (PCNH,Me)PdX (X = Cl, OTf) structures described above. The Pd−O bond length (2.078(7) and 2.080(7) Å from two molecules in the asymmetric unit) is comparable to that found in the handful of other monomeric PdII−OH complexes that have been characterized in the solid state (mean CSD value 2.05 Å).27 For example, the PdII−OH bond length reported for the pincer complex (tBuPCP)Pd(OH)···H2O was 2.095 Å.28

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CONCLUSIONS We have reported the synthesis and characterization of a new unsymmetrical pincer ligand scaffold and complexation of these novel PCN ligands to PdII. With a bulky phosphine and a less sterically hindered pyrazolyl as the arms on the PCN pincer ligand, both mononuclear and dinuclear PdII hydroxide complexes were formed and characterized. Furthermore, a pyrazolyl ring “rollover” activation at the metal center was observed on a pyrazolyl pincer complex bearing an available C− H bond at the 5-position of the pyrazolyl side arm. Observation of this reactivity provides opportunities for the synthesis of new derivatives of the dianionic tridentate (PCC)2− fragment, and it also serves as a warning to those interested in exploiting the “stability” of PCN-ligated structures. Notably, the use of the methylated analogue (PCNMe) inhibits the “rollover” reactivity, allowing for a clean isolation of the mononuclear hydroxide species. Studies to explore the reactivity of the novel mono- and dinuclear hydroxide complexes with small molecules of interest in renewable energy (H2) or carbon capture and sequestration (CO2) research are planned.

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EXPERIMENTAL SECTION

General Considerations and Materials Characterization. All air- and/or moisture-sensitive reactions were performed under an inert atmosphere in flame-dried flasks using standard Schlenk-type techniques or in a glovebox filled with nitrogen. Tetrahydrofuran (THF) was purified by distillation from sodium/benzophenone ketyl, after drying over KOH or used after passage through columns of activated alumina and molecular sieves. Benzene, n-hexane, pentane, CH3CN, and toluene were purified by distillation from sodium/ triglyme benzophenone ketyl or were obtained by means of a MBraun solvent purification system. Pentane and CH2Cl2 were also used after passage through columns of activated alumina and molecular sieves. THF-d8, benzene-d6, and toluene-d8 were dried over sodium/ benzophenone ketyl, condensed in vacuo over activated 4 Å molecular sieves, and degassed by several freeze−pump−thaw cycles prior to use. CD2Cl2 was dried over activated 4 Å molecular sieves or calcium hydride. All other reagents and solvents were used as purchased from commercial suppliers. 1H, 13C{1H}, and 31P{1H} NMR spectra were obtained on a Bruker Avance 700, Bruker Avance 500, Bruker Avance DRX-400, or Bruker Avance 300 MHz instrument. Chemical shifts are reported in ppm (δ) relative to TMS, referenced to the chemical shifts of residual solvent resonances (1H and 13C), and coupling constants are given in Hz. 31P{1H} NMR spectra are referenced to an external 85% H3PO4 sample (0 ppm). The N, C, H elemental analyses were carried out at ICCOM by means of a Carlo Erba Model 1106 elemental analyzer or at the CENTC Elemental Analysis Facility at the University of Rochester. The GC/MS analyses were performed on a Shimadzu QP2010S apparatus equipped with a column identical with that used for GC analysis.

Figure 10. ORTEP representation of the (PCNMe)Pd(OH) complex 9b, with ellipsoids shown at 50% probability. Hydrogen atoms and tBu groups on the pincer ligands are omitted for clarity. The Pd−HO··· H2 O hydrogen bond between the hydroxyl group and the crystallization water molecule is depicted by a yellow dotted line. H


Article

Organometallics X-ray Diffraction Data. X-ray diffraction intensity data were collected on an Oxford Diffraction XcaliburPX (4a,b and 6a) equipped with a CCD area detector or on a Bruker APEX II (7a,b, 8a,b, 9b, and 10a) diffractometer using Mo Kα radiation (λ = 0.71073 Å) at low temperature (either T = 100 or 120 K, see Table S1 in the Supporting Information). The data set was integrated and scaled using SAINT and SADABS within the APEX2 software package by Bruker.29 The program used for the data collection was CrysAlis CCD 1.171.30 Data reduction was carried out with the program CrysAlis RED 1.171,31 and the absorption correction was applied with the program ABSPACK 1.17. Direct methods implemented in Sir9732 were used to solve the structures, and the refinements were performed by full-matrix least squares against F2 implemented in SHELX97.33 All of the nonhydrogen atoms were found from Fourier syntheses of electron density and were refined anisotropically, while the hydrogen atoms were fixed in calculated positions and refined isotropically with the thermal factor depending on the atom to which they are bound (riding model), with C···H distances in the range 0.95−1.00 Å. The geometrical calculations were performed by PARST97.34 The details of crystallographic, collection, and refinement data are shown in Table S1. Molecular plots were produced by the program ORTEP3.35 CCDC 1048866 (4a), 1048867 (4b), 1048868 (6a), 1061228 (7a), 1061229 (7b), 1061230 (8a), 1061231 (8b), 1061232 (9b), and 1061233 (10a) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via ccdc.cam.ac.uk/community/ requestastructure. Computational Details. Density functional theory (DFT) calculations were performed using the Gaussian09 program (revision C.01).36 Model structures were optimized with a M06 functional37 using the SDD/MWB10 pseudopotential and related basis set38 on the palladium and phosphorus atoms plus a 6-31G* basis set on all the other atoms. Introduction of diffuse functions is essential to well reproduce conformational equilibria and experimental electron affinities.39 An extra d-type polarization function for P and an extra f-type function for Pd were added to the standard set.40 Gibbs energy calculations to infer relative thermodynamic stabilities were carried out on the real system. The initial guess geometry for the optimization was obtained by starting from the XRD structure of the dimeric species 10a. IRC analysis41 was performed to find the two minima linked by the related transition structure. When IRC calculations failed to reach the minima, geometry optimizations from the initial phase of the IRC path were performed. Frequency calculations were made on all the optimized structures, to characterize the stationary points as minima or TSs and calculate zero-point energies, enthalpies, entropies, and gasphase Gibbs energies at 298 K. Evaluation of the solvent effects was performed through a continuum modeling of the reaction medium. Bulk solvent effects (THF, θ = 7.42) were expressed through the SMD Continuum Model,42 with the same basis set used for the gas phase optimizations. The Gibbs energy in solution was calculated according to the following simplified equation: GTHF = Ggas + (ETHF − Egas). Synthesis of (3-(1H-Pyrazol-1-yl)phenyl)methanol (1a). To a solution of (3-bromophenyl)methanol (1.00 g, 5.34 mmol) in Nmethylpyrrolidinone (NMP, 32 mL) were added K2CO3 (1.47 g, 10.68 mmol), CuI (0.10 g, 0.53 mmol), and pyrazole (0.38 g, 5.61 mmol) in sequence. The reaction mixture was placed in a microwave apparatus (CEM Discover) and heated at 210 °C for 5 h while a constant irradiation power was maintained (250 W). The mixture was then cooled to room temperature and filtered on a Celite pad to remove all suspended residues, and AcOEt (50 mL) was used to wash the filter. The collected organic phase was evaporated in vacuo to give a viscous residue that was purified by flash chromatography (silica gel, petroleum ether/AcOEt 60/40) to give 1a as a pale yellow oil (0.73 g, yield 78.6%). 1H NMR (300 MHz, CDCl3, 293 K): δ 3.80 (br s, 1H, CH2OH), 4.64 (s, 2H, CH2OH), 6.42 (m, 1H, CH), 7.18 (m, 1H, CH Ar), 7.32 (m, 1H, CH Ar), 7.50 (m, 1H, CH Ar), 7.60 (br s, 1H, CH Ar), 7.66 (m, 1H, CH), 7.85, (m, 1H, CH). 13C{1H} NMR (75 MHz, CDCl3, 293 K): δ 64.2 (s), 107.6 (s), 117.6 (s), 118.1 (s), 124.8 (s), 127.1 (s), 129.4 (s), 140.0 (s), 140.9 (s), 142.9 (s). Anal. Calcd for

C10H10N2O (174.20): C, 68.95; H, 5.79; N, 16.08. Found: C, 69.05; H, 5.84; N, 16.17. Synthesis of (3-(5-Methyl-1H-pyrazol-1-yl)phenyl)methanol (1b). To a stirred solution of 3-(5-methyl-1H-pyrazol-1-yl)benzaldehyde (0.415 g, 2.22 mmol) in dry and degassed EtOH (10 mL) was added NaBH4 (0.126 g, 3.34 mmol) in one portion. The reaction mixture was stirred at room temperature for 1 h before being quenched with water (10 mL). The mixture was then extracted with AcOEt (3 × 20 mL), and the combined organic layers were dried over Na2SO4. Filtration and solvent removal under reduced pressure gave the crude material, which was purified by flash chromatography (silica gel; petroleum ether/AcOEt 60/40) to give 1b as an off-white oil (0.367 g, yield 87.8%). 1H NMR (300 MHz, CD2Cl2, 293 K): δ 2.31 (s, 3H, CH3), 2.94 (br s, 1H, CH2OH), 4.65 (s, 2H, CH2OH), 6.19 (m, 1H, CH), 7.33−7.28 (2H, CH Ar), 7.44−7.37 (2H, CH Ar), 7.51 (m, 1H, CH Ar). 13C{1H} NMR (75 MHz, CD2Cl2, 293 K): δ 12.1 (s), 64.0 (s), 106.7 (s), 123.0 (s), 123.4 (s), 125.6 (s), 128.8 (s), 138.9 (s), 139.6 (s), 139.9 (s), 142.9 (s). Anal. Calcd for C11H12N2O (188.23): C, 70.19; H, 6.43, N, 14.88. Found: C, 70.39; H, 6.65; N, 14.93. General Procedure for Bromination of the Benzyl Alcohol Intermediates 1a,b: Synthesis of 1-(3-(Bromomethyl)phenyl)1H-pyrazole (2a) and 1-(3-(Bromomethyl)phenyl)-5-methyl1H-pyrazole (2b). To a stirred solution of the selected alcohol (1a or 1b) intermediate (5 mmol) and PPh3 (1.97 g, 7.5 mmol) in dry and degassed CH2Cl2 (30 mL) was added Br3CCO2Et (0.93 mL, 5 mmol) dropwise under an N2 atmosphere. After 30 min of stirring at room temperature, the reaction mixture was quenched with water and the formed layers were separated. The aqueous phase was washed with CH2Cl2 (3 × 15 mL), and the combined organic extracts were dried over Na2SO4. After solvent removal the resulting semisolid material was washed with hot hexane (3 × 15 mL) and the organic phases were separated from PPh3 (and OPPh3) residues upon filtration. The crude residue was then purified by flash chromatography (silica gel; petroleum ether/AcOEt 80/20) to give 2a,b as off-white viscous oils in 68.5% and 67.7% isolated yields, respectively. Compound 2a. 1H NMR (300 MHz, CDCl3, 293 K): δ 4.52 (s, 2H, CH2Br), 6.47 (m, 1H, CH), 7.30 (m, 1H, CH Ar), 7.42 (t, 1H, 3JHH = 7.8 Hz, CH Ar), 7.60 (m, 1H, CH Ar), 7.73 (m, 1H, CH), 7.77 (br s, 1H, CH Ar), 7.93, (m, 1H, CH). 13C{1H} NMR (75 MHz, CDCl3, 293 K): δ 32.6 (s), 107.8 (s), 118.9 (s), 119.8 (s), 126.7 (s), 126.9 (s), 129.9 (s), 139.3 (s), 140.0 (s), 141.2 (s). Anal. Calcd for C10H9BrN2 (237.10): C, 50.66; H, 3.83; N, 11.82. Found: C, 50.78; H, 3.89; N, 11.90. Compound 2b. 1H NMR (300 MHz, CD2Cl2, 293 K): δ 2.36 (s, 3H, CH3), 4.56 (s, 2H, CH2Br), 6.21 (m, 1H, CH), 7.48−7.37 (3H, CH Ar), 7.54−7.52 (2H, CH Ar). 13C{1H} NMR (75 MHz, CD2Cl2, 293 K): δ 12.3 (s), 32.7 (s), 107.1 (s), 124.2 (s), 125.2 (s), 127.8 (s), 129.3 (s), 138.7 (s), 139.1 (s), 139.8 (s), 140.4 (s). Anal. Calcd for C11H11BrN2 (251.12): C, 52.61; H, 4.42; N, 11.16. Found: C, 52.73; H, 4.56; N, 11.22. General Procedure for Phosphination of the Bromine Intermediates 2a,b: Synthesis of 1-(3-((Di-tertbutylphosphino)methyl)phenyl)-1H-pyrazole (3a) and 1-(3((Di-tert-butylphosphino)methyl)phenyl)-5-methyl-1H-pyrazole (3b). To a solution of tBu2PH (2.5 mmol) in dry and degassed acetone (10 mL) was added an acetone solution (15 mL) of the the selected bromine (2a or 2b) derivative (2 mmol) in one portion. The resulting mixture was refluxed under an N2 atmosphere for 12 h. Afterward, the cooled solution was treated with 25 mL of dry and degassed pentane, causing the precipitation of the HBr phosphine adduct (3a·HBr or 3b·HBr). The collected white solid was washed with degassed pentane (3 × 15 mL) before being dissolved in 15 mL of degassed H2O and treated with 12 mL of a degassed saturated NaOAc solution. The aqueous phase was then extracted with dry and degassed Et2O (3 × 20 mL), and the collected (colorless) organic phases were dried over Na2SO4. Solvent removal gave 3a,b in the form of an analytically pure white solid (for 3a) and off-white viscous oil (for 3b) in 88.4% and 86.8% isolated yields, respectively. Compound 3a. 31P{1H} NMR (121 MHz, CD2Cl2, 293 K): δ 35.2 (s). 1H NMR (300 MHz, CD2Cl2, 293 K): δ 1.18 (d, 3JPH = 10.8 Hz, I


Article

Organometallics 18H, P-C(CH3)3), 2.94 (d, 2JPH = 2.8 Hz, 2H, Ar-−CH2-P), 6.48 (m, 1H, CH), 7.38−7.30 (2H, CH Ar), 7.48 (m, 1H, CH Ar), 7.69 (m, 1H, CH), 7.75 (m, 1H, CH Ar), 7.98, (m, 1H, CH). 13C{1H} NMR (75 MHz, CD2Cl2, 293 K): δ 28.5 (d, JPC = 24.7 Hz, Ar-CH2-P), 29.5 (d, JPC = 13.2 Hz, P-C(CH3)3), 31.6 (d, 1JPC = 22.7 Hz, P-C(CH3)3), 107.2 (s), 115.8 (s), 120.1 (d, JPC = 9.3 Hz), 126.6 (s), 127.6 (d, JPC = 8.5 Hz), 129.0 (s), 140.0 (s), 140.7 (s), 143.8 (d, JPC = 13.0 Hz). Anal. Calcd for C18H27N2P (302.39): C, 71.49; H, 9.00, N, 9.26. Found: C, 71.60; H, 9.21, N, 9.30. Compound 3b. 31P{1H} NMR (121 MHz, CD2Cl2, 293 K): δ 35.7 (s). 1H NMR (300 MHz, CD2Cl2, 293 K): δ 1.16 (d, 3JPH = 10.8 Hz, 18H, P-C(CH3)3), 2.34 (s, 3H, CH3), 2.92 (d, 2JPH = 2.8 Hz, 2H, ArCH2-P), 6.20 (m, 1H, CH), 7.24−7.20 (m, 1H, CH Ar), 7.38−7.34 (2H, CH Ar), 7.45 (br s, 1H, CH Ar), 7.53 (m, 1H, CH). 13C{1H} NMR (75 MHz, CD2Cl2, 293 K): δ 12.2 (s), 28.3 (d, JPC = 24.7 Hz, Ar-CH2-P), 29.4 (d, JPC = 13.2 Hz, P-C(CH3)3), 31.7 (d, 1JPC = 22.7 Hz, P-C(CH3)3), 106.5 (s), 121.6 (s), 125.8 (d, JPC = 9.2 Hz), 128.6 (s), 128.7 (d, JPC = 8.4 Hz), 138.7 (s), 139.4 (s), 143.4 (d, JPC = 12.7 Hz). Anal. Calcd for C19H29N2P (316.42): C, 72.12; H, 9.24; N, 8.85. Found: C, 72.33; H, 9.40; N, 8.92. General Procedure for the Synthesis of the Chloride Complexes 4a,b. To a stirred solution of the selected PCNR ligand (R = H, 3a; R = Me, 3b) (0.5 mmol) in dry and degassed toluene (3 mL) was added a suspension of Pd(COD)Cl2 (0.50 mmol) in dry and degassed toluene (4 mL) in one portion. The reaction mixture was stirred at reflux of solvent for 4 h, and then it was cooled to room temperature. Afterward, the solvent was removed under vacuum to give a crude mixture as a yellow pale semisolid material. The crude sample was washed with pentane and filtered to afford analytically pure white crystals of 4a,b in 90% and 93% isolated yields, respectively. For both palladium compounds, crystals suitable for X-ray diffraction analysis were growth from concentrated acetone solutions. (PCNH)PdCl (4a). 31P{1H} NMR (161 MHz, CD2Cl2, 293 K): δ 95.0 (s). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 1.48 (d, 3JPH = 14.3 Hz, 18H, P-C(CH3)3), 3.41 (d, 2JPH = 9.3 Hz, 2H, Ar-CH2-P), 6.55 (m, 1H, CH), 7.06−7.15 (3H, CH Ar), 7.98−8.02 (2H, CH Ar). 13 C{1H} NMR (100 MHz, CD2Cl2, 293 K): δ 29.2 (d, 2JPC = 4.3 Hz, P-C(CH3)3), 35.3 (d, 1JPC = 31.6 Hz, Ar-CH2-P), 35.6 (d, 1JPC = 19.7 Hz, P-C(CH3)3), 107.1 (s), 110.1 (s), 122.5 (d, 3JPC = 20.4 Hz), 125.7 (s), 126.0 (s), 139.3 (s), 143.7 (s), 148.6 (s), 150.3 (d, 3JPC = 14.0 Hz). Anal. Calcd for C18H26ClN2PPd (443.26): C, 48.77; H, 5.91; N, 6.32. Found: C, 48.82; H, 5.97; N, 6.39. (PCNMe)PdCl (4b). 31P{1H} NMR (161 MHz, CD2Cl2, 293 K): δ 93.3 (s). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 1.48 (d, 3JPH = 14.2 Hz, 18H, P-C(CH3)3), 2.71 (s, 3H, CH3), 3.39 (d, 2JPH = 9.4 Hz, 2H, Ar-CH2-P), 6.29 (m, 1H, CH), 7.06 (m,1H, CH), 7.13 (m,1H, CH), 7.23 (m, 1H, CH Ar), 7.91 (m, 1H, CH Ar). 13C{1H} NMR (100 MHz, CD2Cl2, 293 K): δ 13.9 (s, CH3), 28.8 (d, 2JPC = 4.3 Hz, PC(CH3)3), 34.9 (d, 1JPC = 28.7 Hz, Ar-CH2-P), 35.0 (d, 1JPC = 17.4 Hz, P-C(CH3)3), 108.0 (s), 111.3 (s), 121.6 (d, 3JPC = 20.4 Hz), 125.0 (s), 138.4 (s), 140.1 (s), 144.9 (s), 149.0 (s), 150.1 (d, 3JPC = 14.4 Hz). Anal. Calcd for C19H28ClN2PPd (457.29): C, 49.90; H, 6.17; N, 6.13. Found: C, 49.93; H, 6.21; N, 6.15. [(PCNH)Pd(MeCN)](BF4) (5a). To a stirred solution of 4a (0.100 g, 0.23 mmol) in dry and degassed CH2Cl2 (5 mL) were added dry and degassed CH3CN (23 μL, 0.44 mmol) and AgBF4 (0.065 g, 0.33 mmol) in sequence, under a nitrogen atmosphere. The resulting mixture was stirred at room temperature for 5 h, and then the solution was filtered via cannula and concentrated to approximately threefourths of its initial volume. To this concentrated solution was added, drop by drop, dry and degassed pentane until a precipitate formed. The final white solid was washed with several portions of pentane and dried under vacuum for 1 h (0.090 g, 75% isolated yield). 31P{1H} NMR (161 MHz, CD2Cl2, 293 K): δ 98.2 (s). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 1.41 (d, 3JPH = 14.7 Hz, 18H, P-C(CH3)3), 2.51 (s, 1H, CH3CN), 3.43 (d, 2JPH = 9.6 Hz, 2H, Ar-CH2-P), 6.64 (m, 1H, CH), 7.07 (m, 1H, CH Ar), 7.13 (m, 1H, CH Ar), 7.20 (m, 1H, CH Ar), 7.90 (m, 1H, CH Ar), 8.07 (m, 1H, CH Ar). 13C{1H} NMR (100 MHz, CD2Cl2, 293 K): δ 2.9 (s, CH3CN), 28.6 (d, JPC = 4.2 Hz, PC(CH3)3), 33.7 (d, JPC = 31.2 Hz, Ar-CH2-P), 35.5 (d, JPC = 17.7 Hz,

P-C(CH3)3), 107.8 (s), 110.6 (s), 122.7 (d, JPC = 21.0 Hz), 126.8 (s), 126.9 (s), 140.5 (s), 143.3 (s), 143.4 (s), 150.4 (d, JPC = 13.5 Hz). 11 1 B{ H} NMR (128 MHz, CD2Cl2, 293 K): δ −1.0. Anal. Calcd for C20H29BF4N3PPd (535.66): C, 44.84; H, 5.46; N, 7.84. Found: C, 44.88; H, 5.50; N, 7.80. {[(PCNH)Pd]2(μ-Cl)}(BF4) (6a). To a stirred solution of 4a (0.100 g, 0.23 mmol) in dry and degassed CH2Cl2 (10 mL) was added 0.5 equiv of AgBF4 (0.022 g, 0.11 mmol) under a nitrogen atmosphere. The resulting mixture was stirred at room temperature for 5 h, and then the solution was filtered via cannula and concentrated to approximately one-third of its initial volume. To this concentrated solution was added, drop by drop, dry and degassed pentane until a precipitate formed. The final pale yellow solid was washed with several portions of pentane and dried under vacuum for 1 h (0.062 g, 60% isolated yield). Crystals suitable for X-ray diffraction were obtained from a concentrated solution of the complex in a mixture of CH2Cl2/pentane cooled at −30 °C. 31P{1H} NMR (121 MHz, CD2Cl2, 293 K): δ 95.1 (s). 1H NMR (300 MHz, CD2Cl2, 293 K): δ 2.17 (d, 3JPH = 14.4 Hz, 36H, P-C(CH3)3), 3.44 (d, 2JPH = 8.7 Hz, 2H, Ar-CH2-P, H), 6.39 (m, 2H, CH), 7.11−7.16 (6H, CH Ar), 8.03−8.09 (4H, CH Ar). 13C{1H} NMR (75 MHz, CD2Cl2, 293 K): δ 28.8 (d, JPC = 3.5 Hz, P-C(CH3)3), 34.0 (d, JPC = 30.3 Hz, Ar-CH2-P), 35.6 (d, JPC = 17.5 Hz, PC(CH3)3), 107.2 (s), 110.6 (s), 122.8 (d, JPC = 20.5 Hz), 126.6 (s), 139.8 (s), 142.9 (s), 144.6 (s), 149.8 (d, JPC = 13.3 Hz). 11B{1H} NMR (96 MHz, CD2Cl2, 293 K): δ −1.1. Anal. Calcd for C36H52BClF4N4P2Pd2 (937.87): C, 46.10; H, 5.59; N, 5.97. Found: C, 46.20; H, 5.63; N, 5.93. (PCNH)Pd(OTf) (7a). AgOTf (33.0 mg, 0.128 mmol) was added to a solution of 4a (51.2 mg, 0.116 mmol) in CH2Cl2 (2 mL) in the absence of light. The suspension was stirred for 4 h and filtered through Celite. Solvent removal yielded 7a as an off-white solid (62.7 mg, 97% yield). Recrystallization from layering pentane on a concentrated solution of 7a in CH2Cl2 yielded clear crystals that were suitable for X-ray crystallography. 31P{1H} NMR (202 MHz, CD2Cl2): δ 94.1 (s). 1H NMR (500 MHz, CD2Cl2): δ 1.43 (d, 3JPH = 14.5 Hz, 18H, P-C(CH3)3), 3.31 (d, 2JPH = 9.6 Hz, 2H, Ar-CH2-P), 6.55 (m, 1H, CH), 7.01 (dd, JHH = 7.6 Hz, 0.9 Hz, 1H, CH), 7.05 (d, 3 JHH = 7.9 Hz, 1H, CH), 7.14 (td, JHH = 7.8, 1.3 Hz, 1H, CH), 7.98− 7.99 (m, 1H, CH), 8.32−8.33 (m, 1H, CH). 13C{1H} NMR (176 MHz, CD2Cl2): δ 28.9 (d, JPC = 4.3 Hz, P-C(CH3)3), 33.0 (d, JPC = 30.6 Hz, Ar-CH2-P), 35.7 (d, JPC = 17.6 Hz, P-C(CH3)3), 107.8 (d, JPC = 3.7 Hz), 110.7 (s), 122.9 (d, JPC = 20.8 Hz), 126.4 (s), 126.6 (s), 139.8 (s), 142.6 (d, JPC = 2.8 Hz), 143.5 (s), 150.7 (d, JPC = 13.7 Hz). Anal. Calcd for C19H26F3N2O3PPdS: C, 40.98; H, 4.71; N, 5.03. Found: C, 40.97; H, 4.56; N, 4.96. [(PCNH)Pd]2(μ-OH)[OTf] (8a). Ground KOH (1.4 mg, 0.0250 mmol, 1 equiv) was added to a solution of 7a (26.7 mg, 0.0479, 2 equiv) in THF (3 mL), and the mixture was stirred for 24 h. The golden mixture was filtered through Celite and concentrated (ca. 1 mL). The product was crystallized in a pentane-vapor diffusion chamber at −30 °C. The isolated golden crystals of 8a were suitable for X-ray crystallography (16.0 mg, 68% yield). 31P(1H} NMR (202 MHz, THF-d8): δ 93.3 (s). 1H NMR (500 MHz, THF-d8): δ −1.48 (s, 1H, Pd−OH), 1.44 (d, 3JPH = 14.1 Hz, 18H, P−C(CH3)3), 1.51 (d, 3 JPH = 14.3 Hz, 18H, P−C(CH3)3), 3.54 (dd, 2JPH = 9.3 Hz, 2JHH = 18.1 Hz, 2H, Ar-CH2-P), 3.58 (dd, 2JPH = 10.2 Hz, 2JHH = 18.1 Hz, 2H, Ar-CH2-P), 6.40−6.42 (m, 2H, CH), 7.07 (dd, JHH = 7.7, 1.1 Hz, 2H, CH), 7.11 (td, JHH = 7.7, 1.3 Hz, 2H, CH), 7.34 (dd, JHH = 7.8, 1.1 Hz, 2H, CH), 8.32−8.33 (m, 2H, CH), 8.45−8.46 (m, 2H, CH). 13C{1H} NMR (176 MHz, THF-d8): δ 29.31 (d, JPC = 4.4 Hz, P−C(CH3)3), 29.52 (d, JPC = 4.4 Hz, P-C(CH3)3), 34.88 (d, JPC = 31.3 Hz, Ar-CH2P), 35.86 (d, JPC = 17.6 Hz, P-C(CH3)3), 36.13 (d, JPC = 17.6 Hz, PC(CH3)3), 107.56 (d, JPC = 3.3 Hz), 111.48 (s), 123.34 (d, JPC = 21.1 Hz), 126.62 (s), 128.11 (s), 140.41 (d, JPC = 1.9 Hz), 144.59 (s), 144.65 (s), 151.13 (d, J PC = 14.1 Hz). Anal. Calcd for C37H53F3N4O4P2Pd2S: C, 45.27; H, 5.44; N, 5.71. Found: C, 46.17; H, 5.51; N, 5.49. (PCNH)Pd(OH) (9a) and (PCNH)Pd(μ-OH)Pd(PCC) (10a). In a medium-walled NMR tube with a resealable Teflon pin, ground KOH (0.8 mg, 0.0143 mmol) was added to a solution of 7a (7.0 mg, 0.0126 J


Article

Organometallics

Hz, 2H, CH), 8.34 (m, 2H, CH). 13C{1H} NMR (176 MHz, THF-d8): δ 14.1 (s, pyz-CH3), 29.30 (d, J = 4.0 Hz, P(C(CH3)3)2), 29.58 (d, J = 4.0 Hz, P(C(CH3)3)2), 34.82 (d, J = 30.6 Hz, CH2P), 35.65 (d, J = 17.6 Hz, P(C(CH3)3)2), 36.00 (d, J = 17.7 Hz, P(C(CH3)3)2), 108.9 (d, J = 1.6 Hz, pyz-C), 112.8 (s, Ar-C), 123.14 (d, J = 20.9 Hz, Ar-C), 126.4 (s, pyz-C), 139.7 (s, pyz-C), 141.8 (s, Ar-C), 145.7 (s, Ar-C), 145.8 (s, Ar-C), 151.48 (d, J = 14.2 Hz, Ar-C). Anal. Calcd for C39H57F3N4O4P2Pd2S: C, 46.39; H, 5.69; N, 5.55. Found: C, 47.02; H, 5.70; N, 5.26. (PCNMe)Pd(OH) (9b). To a solution of 4a (30.0 mg, 0.0656 mmol) in THF (5 mL) was added an excess of ground KOH (15.2 mg, 0.271 mmol). The mixture was sonicated for 2.5 h and then left for 20 h at room temperature. The mixture was filtered through Celite and concentrated to ca. 1 mL. The golden solution was layered with pentane and cooled to −30 °C to yield tan crystals of 9b suitable for X-ray diffraction (20.3 mg, 71% yield). 31P(1H} NMR (121 MHz, THF-d8): δ 90.2 ppm (s). 1H NMR (500 MHz, THF-d8): δ −1.97 (s, 1H, Pd-OH), 1.43 (d, 3JPH = 13.7 Hz, 18H, P-C(CH3)3), 2.66 (s, 3H, CH3), 3.36 (d, 2JPH = 9.2 Hz, 2H, Ar-CH2-P), 6.25 (m, 1H, CH), 6.91−6.93 (m, 2H, CH), 7.17 (dd, 3JHH = 6.1, 2.8 Hz, 1H, CH), 7.79 (m, 1H, CH). 13C{1H} NMR (126 MHz, THF-d8): δ 14.07 (s, pyzCH3), 29.40 (d, J = 5.0 Hz, P(C(CH3)3)2), 35.38 (d, J = 16.9 Hz), 36.16 (d, J = 28.9 Hz), 108.1 (s), 111.6 (s), 121.89 (d, J = 20.2 Hz), 124.1 (s), 138.1 (s), 140.2 (s), 146.3 (s), 150.90 (d, J = 14.8 Hz). Anal. Calcd for C19H29N2OPPd: C, 52.00; H, 6.66; N, 6.38. Found: C, 51.89; H, 6.76; N, 6.18.

mmol) in THF-d8 (0.4 mL). The mixture was shaken intermittently until full conversion to 8a was observed by NMR spectroscopy. An excess of ground KOH (1.2 mg, 0.214 mmol) was then added, and the NMR tube was rotated slowly using a NMR tube rotary device over 16 h. Full conversion of 8a to a mixture of 9a (70% yield based on internal standard, hexamethylbenzene) and 10a was observed by NMR spectroscopy. Compound 9a can also be prepared by the addition of D2O (0.1 mL) to a THF-d8 (0.4 mL) solution of 8a (5.0 mg, 8.98 μmol) containing suspended KOH (0.6 mg, 10.7 μmol). Using this procedure the resulting solution contained only 9a, as determined by solution NMR spectroscopy. Compound 9a. 31P(1H} NMR (202 MHz, THF-d8): δ 91.9 (s). 1H NMR (500 MHz, THF-d8): δ −1.71 (s, 1H, Pd−OH), 1.43 (d, 3JPH = 13.9 Hz, 18H, P-C(CH3)3), 3.38 (d, 2JPH = 9.2 Hz, 2H, Ar-CH2-P), 6.49 (m, 1H, CH), 6.92−6.95 (m, 2H, CH), 7.15 (m, 1H, CH), 8.05 (m, 1H, CH), 8.30 (m, 1H, CH). 1H NMR (700 MHz, THF-d8/ D2O): δ 1.38 (d, 3JPH = 14.1 Hz, 18H, P-C(CH3)3), 3.36 (d, 2JPH = 9.3 Hz, 2H, Ar-CH2-P), 6.59−6.56 (m, 1H, CH), 6.93 (d, 3JHH = 7.5 Hz, 1H, CH), 6.98 (t, 3JHH = 7.6 Hz, 1H, CH), 7.18 (d, 3JHH = 7.8 Hz, 1H, CH), 8.08 (m, 1H, CH), 8.32 (m, 1H, CH). 13C{1H} NMR (176 MHz, THF-d8/D2O): δ 29.47 (d, JPC = 4.8 Hz, P-C(CH3)3), 35.61 (d, JPC = 31.0 Hz, Ar-CH2-P), 35.75 (d, JPC = 17.3 Hz, P-C(CH3)3), 107.98 (d, JPC = 3.5 Hz), 110.61 (s), 122.57 (d, JPC = 20.4 Hz), 125.72 (s), 127.21 (s), 140.72 (d, JPC = 2.8 Hz), 144.8 (s), 147.85 (s), 150.96 (d, JPC = 14.5 Hz). LRMS (ESI-MS) m/z: [M − OH]+ calcd for C18H26N2PPd 407; found 407.3. Compound 10a. 31P(1H} NMR (120 MHz, THF-d8): δ 71.0 (s, P trans to C), 91.1 (s, P trans to N). 1H NMR (700 MHz, THF-d8): δ −1.75 (bs, 1H, Pd-OH), 1.36 (d, J = 12.4 Hz, 9H, P−C(CH3)3), 1.39 (d, J = 12.5 Hz, 9H, P−C(CH3)3), 1.44 (d, J = 13.8 Hz, 9H, PC(CH3)3), 1.51 (d, J = 14.2 Hz, 9H, P-C(CH3)3), 3.08 (dd, 2JPH = 8.9 Hz, 2JHH = 17.4 Hz, 1H, Ar-CH2-P), 3.12 (dd, 2JPH = 8.3 Hz, 2JHH = 17.4 Hz, 1H, Ar-CH2-P), 3.42 (dd, 2JPH = 9.3 Hz, 2JHH = 17.9 Hz, 1H, Ar-CH2-P), 3.49 (dd, 2JPH = 9.3 Hz, 2JHH = 17.9 Hz, 1H, Ar-CH2-P), 6.11−6.12 (m, 1H, CH), 6.31−6.32 (m, 1H, CH), 6.60 (d, J = 7.6 Hz, 1H, CH), 6.74 (td, J = 7.6, 1.1 Hz, 1H, CH), 6.90−6.91 (m, 1H, CH), 6.91−6.93 (m, 1H, CH), 6.96−6.98 (m, 1H, CH), 7.02 (td, J = 7.7, 1.0 Hz, 1H, CH), 7.16−7.18 (m, 1H, CH), 8.17−8.19 (m, 1H, CH), 8.74−8.75 (m, 1H, CH). (PCNMe)Pd(OTf) (7b). AgOTf (42.1 mg, 0.164 mmol) was added to a solution of (PCNMe)PdCl (75.0 mg, 0.164 mmol) in CH2Cl2 (3 mL) in the absence of light. The suspension was stirred for 5 h and filtered through Celite. Layering pentane on the CH2Cl2 solution at room temperature gave clear light yellow crystals of 7b suitable for Xray crystallography (81.8 mg, 87% yield). 31P{1H} NMR (202 MHz, CD2Cl2): δ 92.1 ppm (s). 1H NMR (500 MHz, CD2Cl2): δ 1.42 (d, 3 JPH = 14.5 Hz, 18H, P-C(CH3)3), 2.68 (s, 3H, pyz-CH3), 3.29 (d, 2JPH = 9.59 Hz, 2H, Ar-CH2-P), 6.28 (s, 1H, CH), 7.01 (d, 3JHH = 7.11 Hz, 1H, CH), 7.13 (t, 3JHH = 7.11 Hz, 1H, CH), 7.19 (d, 3JHH = 7.51 Hz, 1H, CH), 8.22 (s, 1H, CH). 13C{1H} NMR (126 MHz, CD2Cl2): δ 14.56 (s, pyz-CH3), 28.96 (d, 2JPC = 3.9 Hz, P(C(CH3)3)2), 32.96 (d, 1 JPC = 29.8 Hz, P(C(CH3)3)2), 35.49 (d, 1JPC = 17.3 Hz, CH2P), 109.3 (d, JPC = 2.75 Hz), 112.3 (s), 122.5 (d, JPC = 20.8 Hz), 126.4 (s), 140.6 (d, JPC = 2.1 Hz), 140.9 (s), 141.9 (s), 145.2 (s), 150.9 (d, JPC = 13.8 Hz). Anal. Calcd for C20H28F3N2O3PPdS: C, 42.08; H, 4.94; N, 4.91. Found: C, 42.06; H, 4.81; N, 4.75. [(PCNMe)Pd]2(μ-OH)[OTf] (8b). To a solution of (PCNMe)Pd(OTf) (14.5 mg, 0.0254 mmol) in THF (3 mL) was added ground KOH (1.0 mg, 0.0178 mmol). The solution was stirred for 16 h, after which it was filtered through a Teflon filter. The solution was concentrated (ca. 1 mL), and recrystallization was accomplished by slow diffusion of pentane into the solution at −30 °C. The dark yellow crystals were washed with water (3 × 1 mL) and dried under vacuum, giving 8b (11.8 mg, 92% yield). 31P{1H} NMR (202 MHz, THF-d8): δ 91.6 ppm (s). 1H NMR (700 MHz, THF-d8): δ −1.46 (s, 1H, PdOH), 1.43 (d, 3JPH = 14.0 Hz, 18H, P-C(CH3)3), 1.49 (d, 3JPH = 14.2 Hz, 18H, P-C(CH3)3), 2.67 (s, 3H, CH3), 3.54 (dd, 2JHH = 18.1, 2JPH = 10.1 Hz, 2H, Ar-CH2-P), 3.58 (dd, 2JHH = 18.1, 2JPH = 10.0 Hz, 2H, Ar-CH2-P), 6.20 (m, 2H, CH), 7.10 (dd, 3JHH = 7.7 Hz, 1.4 Hz, 2H, CH), 7.12 (td, 3JHH = 7.6, 1.1 Hz, 2H, CH), 7.30 (dd, 3JHH = 7.7, 1.4

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ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, and CIF and XYZ files giving selected crystallographic parameters of the solved XRD structures, NMR spectra of all the described compounds, and Cartesian coordinates and absolute GTHF energy values of the optimized structures of 9a, 9a′, TSrot, TS1, 10a···H2O, and 11a. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00355.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for R.A.K.: [email protected]. *E-mail for G.G.: [email protected]. *E-mail for K.I.G.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.L., A.R., and G.G. thank the Fondazione Cariplo (“Crystalline Elastomers” project), the Groupe de Recherche International (GDRI) “Homogeneous Catalysis for Sustainable Development”, and the COST action CM1006: “EUFEN: European F-Element Network” for supporting this work. CREA (Centro Ricerche Energia e Ambiente) in Colle Val d’Elsa (Siena, Italy) is also acknowledged for computational resources. The work done at the University of Washington and the University of New Mexico was supported by the Department of Energy (DEFG02-06ER15765).

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REFERENCES

(1) For general reviews and books, see: (a) Pincer and Pincer-Type Complexes: Application in Organic Synthesis and Catalysis; Szabo, K. J., Wendt, O. F., Eds.; Wiley-VCH: Weinheim, Germany, 2014. (b) Organometallic Pincer Chemistry; van Koten, G., Milstein, D., Eds.; Springer: Heidelberg, Germany, 2013; Top. Organomet. Chem. 40. (c) Niu, J.-L.; Hao, X.-Q.; Gong, J.-F.; Song, M.-P. Dalton Trans 2011, 40, 5135−5150. (d) Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res.

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Organometallics 2008, 41, 201−213. (e) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759−1792. (f) Singleton, J. T. Tetrahedron 2003, 59, 1837−1857. (2) Roesky, H. W.; Singh, S.; Yusuff, K. K. M.; Maguire, J. A.; Hosmane, N. S. Chem. Rev. 2006, 106, 3813−3843. (3) Wenzel, T. T. Stud. Surf. Sci. Catal. 1991, 66, 545−554. (4) Bailey, W. D.; Parkes, M. V.; Kemp, R. A.; Goldberg, K. I. Reactions of Square Planar d8 Pincer Complexes with Oxygen and Hydrogen. In Pincer and Pincer-Type Complexes: Application in Organic Synthesis and Catalysis; Szabo, K. J., Wendt, O. F., Eds.; Wiley-VCH: Weinheim, Germany, 2014; pp 281−298. (5) The pioneering work on PCP-pincer complexes is: (a) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020−1024. More recent (selected) examples are as follows. PCP type: (b) Roddick, D. M. Top. Organomet. Chem. 2013, 40, 49−88. (c) Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.; Warmuth, R.; Brookhart, M.; KroghJespersen, K.; Goldman, A. S. Organometallics 2009, 28, 5432−5444. (d) Leis, W.; Mayer, H. A.; Kaska, W. C. Coord. Chem. Rev. 2008, 252, 1787−1797. (e) Jensen, C. M. Chem. Commun. 1999, 2443−2449. PNP type: (f) Schneider, S.; Meiners, J.; Askevold, B. Eur. J. Inorg. Chem. 2012, 2012, 412−429. (g) Milstein, D. Top. Catal. 2010, 53, 915−923. POCOP type: (h) Timpa, S. D.; Pell, C.; Zhou, J.; Ozerov, O. V. Organometallics 2014, 33, 5254−5262. (i) Jonasson, K. J.; Wendt, O. F. Chem. - Eur. J. 2014, 20, 11894−11902. (j) Lao, D. B.; Owens, A. C. E.; Heinekey, D. M.; Goldberg, K. I. ACS Catal. 2013, 3, 2391−2396. (k) Pandarus, V.; Zargarian, D. Chem. Commun. 2007, 978−980. (l) Hebden, T. J.; St. John, A. J.; Gusev, D. G.; Kaminsky, W.; Goldberg, K. I.; Heinekey, D. M. Angew. Chem., Int. Ed. 2011, 50, 1873−1876. (6) Some (selected) examples are as follows. CCN/CNN/NCN′ type: (a) Luconi, L.; Klosin, J.; Smith, A. J.; Germain, S.; Schulz, E.; Hannedouche, J.; Giambastiani, G. Dalton Trans. 2013, 42, 16056− 16065. (b) Luconi, L.; Rossin, A.; Motta, A.; Tuci, G.; Giambastiani, G. Chem. - Eur. J. 2013, 19, 4906−4921. (c) Bröring, M.; Kleeberg, C.; Kö hler, S. Inorg. Chem. 2008, 47, 6404−6412. PNN type: (d) Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, L. J. W.; Milstein, D. Nat. Chem. 2011, 3, 609−614. (e) Gnanaprakasam, B.; Milstein, D. J. Am. Chem. Soc. 2011, 133, 1682−1685. (f) Zhang, J.; Balaraman, E.; Leitus, G.; Milstein, D. Organometallics 2011, 30, 5716−5724. (g) Balaraman, E.; Gnanaprakasam, B.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2010, 132, 16756−16758. PCO type: (h) Fulmer, G. R.; Kaminsky, W.; Kemp, R. A.; Goldberg, K. I. Organometallics 2011, 30, 1627−1636. PCS type: (i) Li, J.; Siegler, M.; Lutz, M.; Spek, A. L.; Gebbink, R. J. M. K.; van Koten, G. Adv. Synth. Catal. 2010, 352, 2474−2488. (j) Gagliardo, M.; Selander, N.; Mehendale, N. C.; van Koten, G.; Klein G, R. J. M.; Szabó, K. J. Chem. - Eur. J. 2008, 14, 4800−4809. (7) Examples of PCN type complexes can be found in: (a) Hao, X.Q.; Huang, J.-J.; Wang, T.; Lv, J.; Gong, J.-F.; Song, M.-P. J. Org. Chem. 2014, 79, 9512−9530. (b) Khake, S. M.; Soni, V.; Gonnade, R. G.; Punji, B. Dalton Trans. 2014, 43, 16084−16096. (c) Herbert, D. E.; Ozerov, O. V. Organometallics 2011, 30, 6641−6654. (d) Moreno, I.; SanMartin, R.; Inés, B.; Herrero, M. T.; Domínguez, E. Curr. Org. Chem. 2009, 13, 878−895. (e) Poverenov, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. Organometallics 2005, 24, 5937−5944. (8) Mehendale, N. C.; Sietsma, J. R. A.; de Jong, K. P.; van Walree, C. A.; Klein Gebbink, R. J. M.; van Koten, G. Adv. Synth. Catal. 2007, 349, 2619−2630. (9) As a comparison of the catalytic activities of NCP and NCN pyrazole-based pincers, see: (a) Zhang, X.; Suzuki, S.; Kozaki, M.; Okada, K. J. Am. Chem. Soc. 2012, 134, 17866−17868. (b) Inés, B.; SanMartin, R.; Churruca, F.; Domínguez, E.; Urtiaga, M. K.; Arriortua, M. I. Organometallics 2008, 27, 2833−2839. (c) Gong, J.-F.; Zhang, Y.H.; Song, M.-P.; Xu, C. Organometallics 2007, 26, 6487−6492. (10) (a) Chierotti, M. R.; Rossin, A.; Gobetto, R.; Peruzzini, M. Inorg. Chem. 2013, 52, 12616−12623. (b) Rossin, A.; Bottari, G.; LozanoVila, A. M.; Paneque, M.; Peruzzini, M.; Rossi, A.; Zanobini, F. Dalton Trans. 2013, 42, 3533−3541. (bb) Levina, V. A.; Rossin, A.; Belkova, N. V.; Chierotti, M. R.; Epstein, L. M.; Filippov, O. A.; Gobetto, R.;

Gonsalvi, L.; Lledòs, A.; Shubina, E. S.; Zanobini, F.; Peruzzini, M. Angew. Chem., Int. Ed. 2011, 50, 1367−1370. (c) Rossin, A.; Peruzzini, M.; Zanobini, F. Dalton Trans. 2011, 40, 4447−4452. (d) Fulmer, G. R.; Herndon, A. N.; Kaminsky, W.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2011, 133, 17713−17726. (e) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508−2509. (11) Bailey, W. D.; Kaminsky, W.; Kemp, R. A.; Goldberg, K. I. Organometallics 2014, 33, 2503−2509. (12) (a) Luconi, L.; Rossin, A.; Tuci, G.; Germain, S.; Schulz, E.; Hannedouche, J.; Giambastiani, G. ChemCatChem 2013, 5, 1142− 1151. (b) Luconi, L.; Rossin, A.; Tuci, G.; Tritto, I.; Boggioni, L.; Klosin, J. J.; Theriault, C. N.; Giambastiani, G. Chem. - Eur. J. 2012, 18, 671−687. (c) Luconi, L.; Giambastiani, G.; Rossin, A.; Bianchini, C.; Lledós, A. Inorg. Chem. 2010, 49, 6811−6813. (d) Luconi, L.; Lyubov, D. M.; Bianchini, C.; Rossin, A.; Faggi, C.; Fukin, G. K.; Cherkasov, A. V.; Shavyrin, A. S.; Trifonov, A. A.; Giambastiani, G. Eur. J. Inorg. Chem. 2010, 2010, 608−620. (e) Lyubov, D. M.; Fukin, G. K.; Cherkasov, A. V.; Shavyrin, A. S.; Trifonov, A. A.; Luconi, L.; Bianchini, C.; Meli, A.; Giambastiani, G. Organometallics 2009, 28, 1227−1232. (13) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955− 964. (14) The (few) structurally determined (PCN)PdCl complexes can be found in: (a) Cook, E.; Iwasaki, K.; Masuda, J. D.; Xia, A. Polyhedron 2015, 87, 38−42. (b) Hou, A.-T.; Liu, Y.-L.; Hao, X.-Q.; Gong, J.-F.; Song, M.-P. J. Organomet. Chem. 2011, 696, 2857−2862. (c) Gong, J.-F.; Zhang, Y.-H.; Song, M.-P.; Xu, C. Organometallics 2007, 26, 6487−6492. (d) Motoyama, Y.; Shimozono, K.; Nishiyama, H. Inorg. Chim. Acta 2006, 359, 1725−1730. (15) Some examples are as follows. (a) (tBuPCP)PdCl: Johnson, M. T.; Dzolic, Z.; Cetina, M.; Lahtinen, M.; Ahlquist, M. S. G.; Rissanen, K.; Ohrstrom, L.; Wendt, O. F. Dalton Trans. 2013, 42, 8484−8491. (b) (tBuPCP)Pd(COOH): Johansson, R.; Wendt, O. F. Organometallics 2007, 26, 2426−2430. (c) (tBuPCP)Pd(I3): Johnson, M. T.; Dzolic, Z.; Cetina, M.; Wendt, O. F.; Ohrstrom, L.; Rissanen, K. Cryst. Growth Des. 2012, 12, 362−368. (16) (a) Hartshorn, C. M.; Steel, P. J. Organometallics 1998, 17, 3487−3496. (b) Canty, A. J.; Patel, J.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2000, 607, 194−202. (17) Surprisingly, there are very few examples of crystallographically characterized palladium dimers bridged by only one chlorine atom: (a) Grove, D. M.; van Koten, G.; Ubbels, H. J. C.; Spek, A. L. J. Am. Chem. Soc. 1982, 104, 4285−4286. (b) Fornies, J.; Navarro, R.; Sicilia, V.; Martinez, F.; Welch, A. J. J. Organomet. Chem. 1991, 408, 425−437. (c) Ara, I.; Fornies, J.; Martin, A.; Martin, L. F.; Menjon, B.; Miedes, H. Dalton Trans. 2010, 39, 7301−7309. (18) Bryndza, H. E.; Tam, W. Chem. Rev. 1988, 88, 1163−1188. (19) Cámpora, J.; Palma, P.; del Río, D.; Á lvarez, E. Organometallics 2004, 23, 1652−1655. (20) Some examples are: (a) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2012, 134, 2414−2422. (b) Cao, L.; Jennings, M. C.; Puddephatt, R. J. Dalton Trans. 2009, 5171−5176. (c) Klein, A.; Dogan, A.; Feth, M.; Bertagnolli, H. Inorg. Chim. Acta 2003, 343, 189−201. (21) Joslin, E. E.; Quillian, B.; Gunnoe, T. B.; Cundari, T. R.; Sabat, M.; Myers, W. H. Inorg. Chem. 2014, 53, 6270−6279. (22) (a) Salanouve, E.; Retailleau, P.; Janin, Y. L. Tetrahedron 2012, 68, 2135. (b) Alonso, M. T.; Juanes, O.; de Mendoza, J.; RodríguezUbis, J. C. J. Organomet. Chem. 1994, 484, 19−26. (23) (a) Canty, A. J.; Honeyman, R. T.; Skelton, B. W.; White, A. H. Inorg. Chim. Acta 1986, 114, L39−L42. (b) Canty, A. J.; Minchin, N. J. J. Organomet. Chem. 1982, 226, C14−C16. (c) Canty, A. J.; Honeyman, R. T. J. Organomet. Chem. 1992, 430, 245−257. (d) Canty, A. J.; Honeyman, R. T. J. Organomet. Chem. 1990, 387, 247−263. (24) (a) Maenaka, Y.; Suenobu, T.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 9417−9427. (b) Wiley, J. S.; Oldham, W. J., Jr.; Heinekey, D. M. Organometallics 2000, 19, 1670−1676. L


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Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and

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